There are many kinds of railroad cars for carrying particulate material, be it sand or gravel aggregate, plastic pellets, grains, ores, potash, coal or other granular materials. These materials are not liquid, yet may in some ways tend to flow in a somewhat liquid-like manner. Many of those cars have an upper opening, or accessway of some kind, by which the particulate is loaded, and a lower opening, accessway, or door, by which the particulate material exits the car under the influence of gravity. Clearly, while the inlet opening need not necessarily have a movable door (but may include a cover to discourage contamination of the lading or exposure of the lading to the wind, rain or snow), the outlet opening requires a governor of some kind that is movable between a closed position for retaining the lading while the lading is being transported, and an open position for releasing the lading at the destination. The terminology “flow through” or “flow through railroad car” or “center flow” car, or the like, may sometimes be used for cars of this nature where lading is introduced at the top, and flows out at the bottom.
Consider, for example, a hopper car for transporting sand or gravel aggregate. It may have a converging hopper discharge section that has the shape, generally speaking, of an inverted four sided, truncated pyramid. At the truncated bottom end, there may be a stationary plate and a moving plate, or door. When the moving plate and the stationary plate are brought together, the door is closed. The car is filled with lading, and is hauled to its destination. At the destination, the door or gate is opened, and the lading is allowed to escape from the hopper. The doors are driven by actuators connected to the doors by mechanical transmission elements.